Ear protection devices such as ear plugs or earphones (sometimes called “headphones” or “ear muffs”) are commonly worn by individuals who live, work or play in high-noise environments. For example, an operator of stationary or mobile powered equipment (e.g., bulldozers, jackhammers, snowblowers or compressors) having motors or engines may wear ear plugs that may be inserted into an inner ear, in whole or in part, or earphones defining circumaural chambers that encompass all or portions of an outer ear. Such plugs may be formed from foam, rubber or other sound-blocking or sound-absorbing materials that block or attenuate sound waves and reduces the sound pressure levels (or intensities) of such waves which ultimately enter the operator's ear. Likewise, earphones may be constructed with one or more layers of sound-blocking or sound-absorbing materials.
Recently, a number of technological advances have been incorporated into modern ear protection devices. For example, ear plugs or earphones may include one or more miniature speakers that enable sounds to be played at low volumes, and in close proximity, to the inner ear or the outer ear, by a media player or other like device. Thus, some ear plugs or ear phones may be used to passively block or attenuate unwanted sounds within a vicinity of a user while reproducing or broadcasting wanted sounds to the user. Additionally, some ear plugs and earphones are now equipped with noise-cancelling technologies that may actively reduce or minimize the sound pressure levels and/or frequencies of noises that enter a user's ears. For example, noise-cancelling earphones may include one or more ear chambers having microphones for capturing information or data regarding external narrowband noises, computer components that determine intensities and/or frequencies of the noises, and speakers that emit “anti-noise,” or cancelling signals that are substantially equal in intensity to the noises and perfectly out-of-phase (e.g., one hundred eighty degrees out-of-phase, or of reverse polarity), in order to effectively address or cancel the noises.
Despite their ability to actively or passively reduce or cancel unwanted narrowband noise, noise-cancellation devices such as ear plugs or earphones have a number of inherent limitations. For example, an active noise-cancellation device is unable to distinguish between sound signals based on their content. Such devices merely detect and characterize noises in terms of their sound pressure levels and frequencies, and treat all active or ambient noises identically, e.g., by generating and emitting anti-noises that are intended to reduce the effects of such noises, or cancel the effects of such noises entirely. Because noise-cancellation devices act without regard to contents of sound signals, however, such devices block out all conversational speech, all alarms or notifications, or all machinery sounds, and are unable to identify relevant speech, relevant alarms or relevant machinery sounds, or to allow any relevant sounds to pass therethrough to a user. As a result, in audibly cluttered environments, workers who must be able to engage in conversation or listen for alarms, notifications or sounds emitted by machinery sometimes forego the use of any ear protection devices, thereby subjecting themselves to increased risks of ear damage or hearing loss.